It may depend on the nature of the yogurt. A good naturally fermented yogurt, without a lot of added ingredients, unpasteurized, will slowly continue the fermentation process. To me this means that it is safe and may be more tart. The big no-no with yogurt is contamination , often from a spoon which has been used for other ingredients or used in your mouth, dipped into the yogurt container, which will turn up with various colored molds when you open it later. Keep it clean and the yogurt will be good for a while past those dates -- which I consider a scientific fiction.